1. Field of the Invention
The present invention relates to a dielectric filter, a duplexer used in a microwave band or the like, and a communication apparatus including the same.
2. Description of the Related Art
FIG. 6 shows an example of a prior art dielectric filter used in a microwave band. In the figure, shaded parts are non-conducting portions where the base material of a dielectric block is exposed.
In the dielectric filter, resonator holes 2a and 2b, and external coupling holes 3a and 3b are disposed to penetrate opposing end faces, which are the upper and lower surfaces shown in FIG. 6, of a dielectric block 1. An inner conductor is formed on each of the inner peripheral surfaces of those holes, and an outer conductor 6 is formed on the outer surfaces of the dielectric block 1. At the upper surface of the dielectric block 1, the non-conducting portions of the outer conductor 6 are disposed around the opening portions of the resonator holes 2a and 2b to separate the inner conductor of the resonator holes 2a and 2b from the outer conductor 6. Thus, the upper surface of the dielectric block 1 is used for the open face of each resonator hole. Meanwhile, at the lower surface of the dielectric block 1, the inner conductors of the resonator holes 2a and 2b are electrically connected, namely, short-circuited to the outer conductor 6, so that the lower surface of the dielectric block 1 is used for the short-circuited face of each of the resonator holes 2a and 2b. In addition, input and output electrodes 7a and 7b separated from the outer conductor 6 are formed extending over the lower and side surfaces of the dielectric block 1. The inner conductors of the external coupling holes 3a and 3b are electrically connected to the outer conductor 6 at the upper surface of the dielectric block 1, and are electrically connected to the input and output electrodes 7a and 7b, respectively, at the lower surface thereof.
In the dielectric filter, the individual resonators corresponding to the resonator holes 2a and 2b make a comb-line coupling by stray capacitance Cs occurring between the non-conducting portions of the outer conductor 6, and the external coupling holes 3a and 3b make an inter-digital coupling with the adjacent resonator holes 2a and 2b, respectively, so as to obtain an external coupling. The degree of coupling between the resonators is determined by adjusting the configuration and size of the non-conducting portion at the open face of each resonator hole to change the value of stray capacitance Cs.
In this way, when a conductor for obtaining a coupling between resonators is formed on the open face of each resonator hole, or when one end face of a resonator hole is an open face so that an external coupling is obtained by an inter-digital coupling between an external coupling hole and a resonator hole, it is necessary to dispose an area having the outer conductor formed and an area devoid of the outer conductor on the one end face of the dielectric block.
In this dielectric filter, the outer conductor on the open face is formed by cutting off specified parts of a pattern-printed conductor or a conductor formed by coating the entire open face.
In the prior art dielectric filter described above, however, since the open face, as the opening end of the resonator hole, is formed by pattern-printing or partially cutting off the outer conductor, there are problems in that the formation of the open face of a resonator hole is complicated and manufacturing procedures are increased so that manufacturing costs rise. Moreover, it is difficult to form the outer conductor of the open-face side in a desired configuration with high precision, which causes problems such as deterioration in filter characteristics and increase in characteristic variations. In other words, in the case of the printing method, deviations in printing patterns and spreading or unevenness of conductive pastes occur, while in the method of partially removing a conductor, removal with high dimensional precision is difficult, with the result that the outer conductor cannot be formed with high precision on the surface on the open-face side.
In addition, it is difficult to set and adjust stray capacitance for coupling between the resonators of the conventional dielectric filter, and a large capacitance cannot be obtained.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter, a duplexer, in which the open face of a resonator hole can easily be formed with a high dimensional precision and a coupling between resonators and an external coupling can easily be adjusted, and a communication apparatus incorporating the same.
One preferred embodiment of the present invention provides a dielectric filter having a dielectric block including a base having a pair of opposing end faces, a plurality of protrusions formed on one end face of the base, a resonator hole penetrating from an end face of each protrusion to the other end face of the base, at least one external coupling hole penetrating between both end faces of the base, an inner conductor formed on each of the inner surfaces of the resonator holes and the at least one external coupling hole, and an outer conductor formed on substantially the entire outer surface of the dielectric block except the end face of each protrusion as an open face of each resonator hole.
In the above described dielectric filter, the axial lengths of the resonator holes may be the same.
In addition, an input and output electrode connected to the inner conductor of the at least one external coupling hole and separated from the outer conductor may be formed extending over the other end face of the base of the dielectric block or both the other end face and a side surface thereof.
Another preferred embodiment of the present invention provides a duplexer including a dielectric block having at least two or more filters formed therein, in which at least one of the filters is the dielectric filter described above.
Yet another preferred embodiment of the present invention provides a communication apparatus including at least one of the dielectric filter and the duplexer described above.
In the dielectric filter or the duplexer having such structures, the resonator holes are formed by penetrating inside the plurality of protrusions formed on the base of the dielectric block, and the external coupling hole is formed by penetrating the base, by which the open face of each resonator hole is protruded in contrast to that of the external coupling hole. As a result, after formation of the conductors including the inner conductor and the outer conductor on the entire surface of the dielectric block, the open face of each resonator hole can easily be formed by grinding the end face of each protrusion to remove the outer conductor formed thereon. Moreover, since the shape of the open face of each resonator is determined by the configuration of the protrusion, that is, by the outside shape of the protrusion and the shape of the resonator hole, a high dimensional accuracy is obtainable.
In this case, the protrusion is formed in a tube shape, and in the proximity to the open face of the resonator hole, the outer conductor formed on the external periphery of the protrusion and the inner conductor of each resonator hole oppose along the longth of the protrusion so that a high stray capacitance contributing to a coupling between the resonators can steadily be obtained. In addition, the stray capacitance can be adjusted over a wide range and the magnitude of an external coupling can be adjusted by changing the configuration of the protrusion.
In other words, in the arrangement of the present invention, the open face of the resonator hole can be formed in a short period of time with high precision, and a desired stray capacitance can steadily be obtained by an easy procedure. Accordingly, a coupling between the resonators and an external coupling can be adjusted with high precision over a wide range.
Furthermore, when the axial lengths of the resonators are made equal, the end faces of the protrusions as the open faces of the resonators can be all surface-ground at one time. As a result, the manufacturing procedure in forming the open faces can be simplified.
Furthermore, on the other end face opposing the end face on which the protrusions are disposed, the input and output electrode separated from the outer conductor is formed to make an inter-digital coupling between the external coupling hole and the adjacent resonator hole so as to obtain a larger external coupling.
Furthermore, since the communication apparatus in accordance with the present invention is formed by including a dielectric filter or a duplexer having the above described characteristics, production cost is reduced and satisfactory characteristics are obtainable.